The invention relates to a monitoring process of a quasi-closed electromagnetic shield comprising emission, inside the shield, of electromagnetic signals and detection by at least one receiver assembly located outside the shield of the electromagnetic signals transmitted through the shield, and to a device for implementing this process.
Shielding techniques of premises, for example by means of a Faraday cage, to protect equipment against electromagnetic interference of external origin are widely used. However, this protection is only properly achieved if the movable contacts designed to provide the electromagnetic tightness at the level of the openings, such as doors and windows, are in good condition.
The inevitable ageing of these contacts makes it necessary to check the quality of the shielding periodically. It is a state-of-the-art technique (first process) to determine the efficiency of a shielding constituted by a quasi-closed electromagnetic shield by measuring outside the latter, by means of a selective measuring device, the electromagnetic field produced by a generator of an A.C. current of predetermined frequency whose output is connected to a frame antenna, the antenna and generator being located inside the shield. A similar device wherein the measuring device is inside the shield, and the antenna-generator assembly outside the shield is also state-of-the-art (second process). The frequencies generally used are comprised between 1 kHz and 100 kHz, for at these frequencies the usual electromagnetic shields do not give good performances. To give an example, the attenuation due to the shield is typically 80 dB at 100 kHz, 60 dB at 10 kHz and 40 dB at 1 kHz. The electromagnetic waves passing through the shield can thus easily be measured outside, the decrease of the attenuation being representative of the quality of the shielding.